Do you have DLC installed ?
Did you delete them and try again to play ?
I have a friend who had the same problem. He deleted the DLC, loaded his game, played for awhile, saved his game, then downloaded again the DLC, and it worked.
it was on Xbox 360.
You know whats simpler? Disconnecting from Live. Much quicker and I don't mid cutscene notifications about CatL0v3r 4978 coming online.